I had difficulties adding an extension to the "Folder Options" dialog in Windows Explorer.
I opened the dialog as usual via the "Tools" menu in Windows Explorer. In the "File Types" tab, I pressed "New", to add a new extension. It worked fine, showing up in the "Registered file types" list. But, when closing the options dialog, and reopened it again, the extension was gone. Strange.
The solution for this was simply to remove the entry in HKEY_CLASSES_ROOT and start all over again.